The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly relates to an air/fuel ratio control system, for an internal combustion engine, which both provides control of the primary fuel-air mixture provided to the intake system of the engine by a mixture supply device, and also further provides control of the secondary air/fuel ratio of the exhaust gases of the engine emitted through the exhaust system thereof, by injection of secondary air into said exhaust system.
Nowadays, in the case of a modern internal combustion engine, it has become common and well known to fit a three way catalytic converter to the exhaust system of such an engine, in order effectively to reduce the levels of undesirable pollutants in the exhaust gases which are being emitted therethrough, before they are vented to the atmosphere. Such a per se well known three way catalytic converter is capable of reducing the levels of uncombusted combustible components such as HC and CO in the exhaust gases, at the same time as reducing the levels of nitrogen oxides (i.e., so called NOx) present in said exhaust gases, and such a three way catalytic converter is very effective in practice. However, in an internal combustion engine which is equipped with such a three way catalytic converter, it is very important that the air/fuel ratio of the exhaust gases which are being passed into the catalytic converter should be controlled within a certain rather narrow range around the stoichiometric air/fuel ratio, in order for the three way catalytic converter effectively to perform its three way function of purifying the exhaust gases not only of HC and CO, but also of NOx. In fact, if the air/fuel ratio of the exhaust gases entering the catalytic converter becomes lower than stoichiometric, i.e. richer than stoichiometric, by more than a certain small amount, then the performance of the three way catalytic converter for purifying the exhaust gases of HC and CO and other unburnt combustible components becomes poor, although the performance of the three way catalytic converter for purifying the exhaust gases of NOx remains quite good. On the other hand, if the air/fuel ratio of the exhaust gases entering the three way catalytic converter becomes higher than stoichiometric, i.e. leaner than stoichiometric, by more than a certain small amount, although the performance of the three way catalytic converter for purifying said exhaust gases of HC and CO, etc., still remains good, its performance for purifying said exhaust gases of NOx becomes rather poor.
Therefore it is normally practiced, in internal combustion engines equipped with such three way catalytic converters, to adjust the air/fuel ratio of the primary air/fuel mixture produced by the carburetor or other mixture supply device and supplied to the intake system of the internal combustion engine, i.e. the primary air/fuel ratio, to be substantially richer, i.e. smaller, than the stoichiometric air/fuel ratio. Thus, the internal combustion engine is operated in somewhat over rich condition. Then, secondary air is further injected into the exhaust manifold or the exhaust system of the internal combustion engine upstream of the three way catalytic converter, and the flow rate of this secondary air is controlled according to a signal produced by an oxygen sensor mounted within said exhaust system downstream of the secondary air injection point therein (and upstream of the three way catalytic converter), in such a way as to ensure that the air/fuel ratio of the exhaust gases entering into the three way catalytic converter, i.e. the secondary air/fuel ratio, is very close to stoichiometric. In other words, the primary air/fuel ratio of the fuel-air mixture provided by the mixture supply device is set to be richer, i.e. smaller, than the minimum air/fuel ratio which, during actual operation of the internal combustion engine, can result in production of exhaust gases which are at the stoichiometric condition, in order that it should always be the case that, whatever be the engine operational conditions, the exhaust gases, as they are emitted by the internal combustion engine, should be in a condition somewhat richer than stoichiometric, and therefore should be capable of being brought to the stoichiometric condition by the addition of secondary air.
However, it is often the case that the basic primary air/fuel ratio of the fuel-air mixture produced by the mixture supply device may become somewhat richer or leaner than anticipated, due to tolerances in the manufacture of the mixture supply device, or due to long term changes in the mixture supplying properties of the mixture supply device. Furthermore, changes in barometric pressure may also significantly affect the air/fuel ratio of the fuel-air mixture produced by the mixture supply device.
When such an unanticipated change in the air/fuel ratio of the mixture produced by the mixture supply device occurs, if this change is a change to the richer, i.e. to a smaller air/fuel ratio, then, although of course due to the injection of secondary air provided by the secondary air injection system into the exhaust gases of the engine before they enter the catalytic converter the secondary air/fuel ratio of the exhaust gases entering the three way catalytic converter is maintained to be in the aforesaid desirable narrow range around the stoichiometric condition, nevertheless the problems arise, first that the fuel economy of the internal combustion engine is deteriorated, and also that the exhaust gases entering the three way catalytic converter contain a rather large amount of uncombusted combustible components such as HC and CO and also rather large corresponding amounts of air, and the production of heat during the catalytic combustion of these components within the three way catalytic converter may well result in the overheating thereof.
On the other hand, if the aforesaid unanticipated change in the primary air/fuel ratio of the primary fuel-air mixture provided by the mixture supply device occurs in the direction of producing a leaner fuel-air mixture, i.e. in the direction of a larger air/fuel ratio, then the drivability of the vehicle incorporating the internal combustion engine is deteriorated, and, further, if the secondary air/fuel ratio of the exhaust gases is made so much leaner than previously that in fact the secondary air/fuel ratio becomes leaner than stoichiometric, then it becomes difficult for the catalytic converter to purify these exhaust gases of nitrogen oxides, because of course it is impossible for the injection of secondary air provided by the secondary air injecting system actually to make the exhaust gas secondary air/fuel ratio smaller.